The invention relates to web accumulators and methods for accumulating and discharging a reserve portion of a continuous web passing through the accumulator. The invention enables continuous operation of processing stations on either or both sides of the accumulator when the speed of the web moving through the processing stations temporarily varies between the two stations. The invention is particularly useful for handling webs that tend to wrinkle or fold over during processing.
In many processing operations involving continuous lengths of web material, there are temporary differences in operating speeds between two adjacent operating stations. For example, in the manufacture of absorbent articles, it is common to unwind raw materials from large supply rolls and conduct them into a converting operation. In such operations, it is desirable to unwind subsequent supply rolls without shutting down the converting operation. The operation is desirably maintained by splicing a new supply web to the end of the expiring supply web. This would require stopping the converting operation unless a reserve portion of the expiring web had been accumulated for continued operation while a new supply roll is prepared. This problem is quite old and is generally solved through the use of web accumulators.
The typical accumulator for such use is the festooning type. Festooning type accumulators typically consist of a set of fixed web rolls and a set of movable web rolls which are moved away from the fixed rolls for accumulating a reserve portion of the web and moved toward the fixed rolls for discharging the accumulated reserve portion of the web.
The web is typically looped alternately from a roll of the first set to a roll of the second set in consecutive order. In these configurations, there is typically about 180 degrees of wrap contact between the rolls and the web to maximize the capacity of the accumulator relative to the length. This configuration generally results in significant tractional forces between the web and the rolls and may not be suitable for webs having extensibility in the cross-machine direction (CD extensible webs) because of wrinkling.
It is also known through prior joint development to accumulate webs having machine direction extensibility (MD extensible webs) by running at tensions below 0.05 pounds per lineal inch of web. The MD extensible web is susceptible to clinging and wrapping as the web passes from one set of rolls to the other set, at least in part, because of the low tension. To address this issue, the spacing between web passes is increased relative to conventional accumulators. To increase the spacing between web passes, the web is alternately looped from two rolls in the first set to two rolls in the second set in consecutive order and the two rolls are spaced apart in the machine direction by 1 to 2.5 times the diameter of the roll. This configuration results in less than 180 degrees of wrap contact between the rolls and the webs. While designed for accumulating MD extensible webs at very low tensions, this apparatus and method have been used to accumulate other webs at tensions up to 0.15 pounds per lineal inch.
However, there still exists a need for a web accumulator and a method of web accumulation adapted for CD extensible webs.